1. Field of the Invention
The present invention relates generally to a Broadband Wireless Access (BWA) communication system, and in particular, to a method for transmitting a traffic indication message by a base station in a BWA communication system using an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA).
2. Description of the Related Art
In a 4th generation (4G) communication system, which is a next generation communication system, active research is being conducted on technology for providing users with services guaranteeing various Qualities-of-Service (QoSs) at a data rate of about 100 Mbps.
A current 3rd generation (3G) communication system generally supports a data rate of about 384 Kbps in an outdoor channel environment having a relatively poor channel environment, and supports a data rate of a maximum of about 2 Mbps in an indoor channel environment having a relatively good channel environment. A Wireless Local Area Network (LAN) system and a Wireless Metropolitan Area Network (MAN) system generally support a data rate of 20 Mbps to 50 Mbps.
Therefore, in the current 4G communication system, active research is being performed on a new communication system securing mobility and QoS for the Wireless LAN system and the Wireless MAN system supporting a relatively high data rate in order to support a high-speed service that the 4G communication system intends to provide.
The Wireless MAN system having wide coverage and supporting a high data rate is suitable for a high-speed communication service. However, because the Wireless MAN system does not consider the mobility of users or subscriber stations (SSs), it never considers a handoff caused by high-speed movement of subscriber stations.
In FIG. 1, a description will be made of a configuration of a communication system employing an IEEE (Institute of Electrical and Electronics Engineers) 802.16a standard, i.e., a standard specification for the Wireless MAN system (hereinafter referred to as an “IEEE 802.16a communication system). More specifically, FIG. 1 is a diagram schematically illustrating a BWA communication system using an OFDM/OFDMA.
However, before a description of FIG. 1 is given, it is well known that the Wireless MAN system, i.e., a BWA communication system, has wider coverage and supports a higher data rate compared with the Wireless LAN system. The IEEE 802.16a communication system refers to a communication system utilizing an OFDM/OFDMA to support a broadband transmission network for a physical channel of the Wireless MAN system.
That is, the IEEE 802.16a communication system refers to a BWA communication system employing OFDM/OFDMA. The IEEE 802.16a communication system, as it applies OFDM/OFDMA to the Wireless MAN system, transmits a physical channel signal using multiple subcarriers, thereby enabling high-speed data transmission.
An IEEE 802.16e communication system is a communication system that considers mobility of subscriber stations in the IEEE 802.16a communication system. Currently, no specification has been provided for the IEEE 802.16e communication system.
As a result, both the IEEE 802.16a communication system and the IEEE 802.16e communication system correspond to a BWA communication system utilizing OFDM/OFDMA, and for convenience, the following description will be made with reference to the IEEE 802.16a communication system. In the description below, a term “mobile station (MS)” or “mobile subscriber station (MSS)” is used to describe a “subscriber station (SS)” that it has mobility.
Referring to FIG. 1, the IEEE 802.16a communication system has a single-cell configuration, and includes a base station (BS) 100 and a plurality of subscriber stations (SSs) 110, 120, and 130, which are controlled by the BS 100. Signal transmission and reception between the BS 100 and the SSs 110, 120, and 130 is achieved using OFDM/OFDMA.
In the IEEE 802.16e communication system, if mobility of subscriber stations is taken into consideration, power consumption of the subscriber stations is an important factor for the system. Therefore, a sleep mode operation and an awake mode operation between the BS and the subscriber stations have been proposed to minimize the power consumption of the subscriber stations.
FIG. 2 is a diagram schematically illustrating a sleep mode operation proposed in the IEEE 802.16e communication system. However, before a description of FIG. 2 is given, it is noted that the sleep mode has been proposed to minimize power consumption of an MSS in an idle interval for which no packet data is transmitted during packet data transmission. That is, in the sleep mode, both the MSS and the BS state transition to the sleep mode, thereby minimizing power consumption of the MSS in the idle interval for which no packet data is transmitted.
Generally, the packet data is generated on a burst-by-burst basis. Therefore, it is unreasonable that an interval for which no packet data is transmitted is identical in operation to an interval for which packet data is transmitted. Accordingly, the sleep mode has been proposed. However, when there is transmission packet data while the BS and the MSS are in the sleep mode state, both the BS and the MSS must simultaneously state transition to an awake mode to exchange the packet data.
The sleep mode has been proposed to minimize not only the power consumption but also interference between channel signals. However, because the packet data is considerably affected by traffic, the sleep mode operation should be performed considering a traffic characteristic and a transmission method of the packet data.
Referring to FIG. 2, reference numeral 211 represents a packet data generation pattern. The packet data generation pattern includes multiple ON-intervals and multiple OFF-intervals. The ON-intervals correspond to burst intervals for which packet data, or traffic, is generated, and the OFF-intervals correspond to idle intervals for which no traffic is generated.
The MSS and the BS state transition to the sleep mode or the awake mode according to the traffic generation pattern, thereby minimizing power consumption of the MSS and removing interference between channel signals.
Reference numeral 213 represents state transition (or mode change) of the BS and the MSS. The state transition pattern includes multiple awake modes and multiple sleep modes. In the awake modes, in which traffic is generated, packet data is exchanged between the BS and the MSS. However, in the sleep modes, when no traffic is generated, no packet data is exchanged between the BS and the MSS.
Reference numeral 215 represents an MSS power level pattern. As illustrated in FIG. 2, a power level of the MSS in the awake mode is defined as ‘K’, and a power level of the MSS in the sleep mode is defined as “M.” Comparing the power level K of the MSS in the awake mode with the power level M of the MSS in the sleep mode, the power level M is much lower than the power level K. That is, in the sleep mode, almost no power is consumed because no packet data is exchanged.
In operation, a MSS should receive a state transition approval from a BS in order to make a state transition to the sleep mode, and the BS transmits packet data after permitting the MSS to make a state transition to the sleep mode.
In addition, the BS should inform that there is packet data to be transmitted to the MSS, during a listening interval of the MSS. In this case, the MSS should awake from the sleep mode and determines if there is packet data to be transmitted from the BS.
If it is determined that there is packet data to be transmitted from the BS, the MSS state transitions to the awake mode and receives the packet data from the BS. However, if it is determined that there is no packet data to be transmitted from the BS, the MSS can return to the sleep mode.
A description will now be made of the parameters required to support the sleep mode operation and the awake mode operation.
(1) Basic Connection Identifier (CID)
A CID proposed in the IEEE 802.16e communication system is illustrated in Table 1, and is used for identifying a connection between a BS and an MSS.
TABLE 1CIDValueDescriptionInitial Ranging0x0000Used by an MSS during initial ranging aspart of network entry processBasic CID0x0001~mPrimary Managementm + 1~2mCIDsTransport CIDs and2m + 1~0xFEFFSecondary ManagementCIDsAdaptive Antenna0xFF00A BS supporting AAS shall use this CIDSystem (AAS) initialwhen allocating a Initial Ranging periodranging CIDfor AAS devicesMulticast Polling CIDs0xFF00~0xFFFEAn MSS may be included in one or moremulticast groups for the purposes ofobtaining bandwidth via polling.These connections have no associatedservice flowBroadcast CID0xFFFFUsed for broadcast information that istransmitted on a downlink to all MSSs
As illustrated in Table 1, the CID has a size of 16 bits, and is generally used for a header of a Medium Access Control (MAC) frame to identify a connection. Alternatively, however, the CID is also used for a MAC Service Data Unit (SDU), like a CID described below with reference to a traffic indication message.
With reference to Table 1, a description will now be made of each CID.                Initial Ranging CID: This is a CID for a ranging request (RNG-REQ) message that an MSS transmits to a BS in order to be allocated a Primary Management CID and a Basic CID, and all MSSs should know a value 0x0000 of the Initial Ranging CID. In an Association process to the BS, the MSS informs the BS of its own MAC address through a ranging request message, such that the BS preferentially maps a MAC address of the MSS, a CID indicating the MSS, for example, a Primary Management CID described below, and a Basic CID.        Primary Management CID: This is a CID used for MAC Management message processing, which should be necessarily performed between a MSS and a BS, and the Primary Management CID is used for identifying the MSS. As illustrated in Table 1, one BS can manage/identify m MSSs.        
Herein, ‘m’ refers to the number of MSSs that can be managed by the BS, and can have a different value according to capacity of the BS. The Primary Management CID is a CID that the MSS acquires by a ranging response (RNG-RSP) message.                Basic CID: This is a CID used for MAC Management message processing, which should be optionally performed between a MSS and a BS, and the Basic CID is used for identifying the MSS. As illustrated in Table 1, the Basic CID covers m MSSs, like the Primary Management CID. In addition, the Basic CID, like the Primary Management CID, is a CID that the MSS acquires by a ranging response message.        Broadcast CID: This is a CID indicating a message that all MSSs should receive and process, and the Broadcast CID has a value 0xFFFF that all MSSs already know.        Multicast Polling CID: This is a CID allocated/released by a Multicast Polling Allocation Request (MPA-REQ) message, and the Multicast Polling CID is used in Multicast Polling Service and can make a total of 253 multicast groups.        Transport CID: This is a CID used for transmission/reception of general user data traffic. The Transport CID is allocated through a Dynamic Service Addition Response (DSA-RSP) message responsive to a BS-Initiated DSA Request (DSA-REQ) message and an MSS-Initiated DSA-REQ message, and the total number of available Transport CIDs is calculated as shown in Equation (1).Maximum Number of Transport CIDs=Total Number (65535) of CIDs−Number (m) of Primary Management CIDs−Number (m) of Basic CIDs−Number (1) of Initial Ranging CIDs−Number (1) of Broadcast CIDs  (1)        Secondary Management CID: This represents a CID for management connection for an upper layer such as Simple Network Management Protocol (SNMP)/Trivial File Transfer Protocol (TFTP), and is allocated by a registration response (REG-RSP) message. The total number of available Secondary Management CIDs falls within the same range as the number of the Transport CIDs, but a part of the Secondary Management CIDs is used within the range.        Adaptive Antenna System (AAS) Initial Ranging CID: This is a CID used for allocating an Initial Ranging period for AAS devices by a BS supporting an AAS.        
The Basic CID is used to identify an MSS by a BS. In addition, the Basic CID is allocated by an RNG-RSP message received from the BS while the MSS is performing an Association process to the BS, i.e., performing ranging. That is, the Basic CID is one of the CIDs that the BS maps to unique MAC addresses of the MSSs on a one-to-one basis. In addition, until the MSS is de-associated, the Basic CID is used for designating only the MSS, and has a unique value only within one BS. Therefore, the Basic CID can be used for designating a particular MSS in one BS.
For the MSS, the BS allocates the 16-bit Basic CID value, and the BS can allocate as many Basic CID values as the maximum number of MSSs that the BS can manage. For example, if the BS can manage m MSS, the Basic CID has a value between 1 and m.
(2) Sleep Interval
The sleep interval can be requested by an MSS and can be allocated by a BS in response to a request from the MSS. The sleep interval represents a time interval for which the MSS maintains the sleep mode until a start of the listening interval, after state transitioning to the sleep mode. That is, the sleep interval is defined as a time for which the MSS stays in the sleep mode.
Even after the sleep interval, the MSS can continuously stay in the sleep mode if there is no transmission data from the BS. In this case, the MSS updates the sleep interval by increasing the sleep interval using an initial-sleep window value and a final-sleep window value.
The initial-sleep window value is an initial minimum value of the sleep interval, and the final-sleep window value is a final maximum value of the sleep interval. In addition, the initial-sleep window value and the final-sleep window value can be represented by a number of frames, and both are allocated by the BS. A more detailed description of the initial-sleep window value and the final-sleep window value will be made herein below.
(3) Listening Interval
The listening interval is a parameter existing in a registration response (REG-RSP) message transmitted from the BS to the MSS in response to a registration request (REG-REQ) message transmitted from the MSS to the BS in a Registration process of the MSS. The listening interval represents a time interval for which the MSS awakes from the sleep mode for a while and receives downlink messages such as a traffic indication (TRF_IND) message in synchronism with a downlink signal from the BS.
The traffic indication message indicates the presence of a traffic message, or packet data, to be transmitted to the MSS, and a detailed description thereof will be made below. That is, the MSS continuously waits for the traffic indication message for the listening interval, and if a Basic CID designating the MSS exists in the traffic indication message (Positive Basic CID), the MSS continuously maintains the awake mode, state transitioning to the awake mode. However, if the listening interval expires while no Basic CID designating the MSS exists in the received traffic indication messages (Negative Basic CID), the MSS state transitions to the sleep mode.
(4) Sleep Interval Update Algorithm
After a state transition to the sleep mode, the MSS determines a sleep interval, regarding a predetermined initial-sleep window value as a minimum sleep mode period. After expiration of the sleep interval, the MSS awakes from the sleep mode, and then state transitions to the listening interval. For the listening interval, the MSS continuously determines if there is packet data to be transmitted from the BS. If it is determined that there is no transmission packet data for the listening interval, the MSS doubles the sleep interval and returns to the sleep mode.
More specifically, for example, if the initial-sleep window value is ‘2’, the MSS sets the sleep interval to 2 frames and remains in the sleep mode during the 2 frames. After expiration of the 2 frames, the MSS awakes from the sleep mode and determines if the traffic indication message is received from the BS.
If it is received the traffic indication message for the listening interval, the MSS determines if a Basic CID exists in the received traffic indication message. If it is determined that the Basic CID does not exist in the received traffic indication message, the MSS sets the sleep interval to 4 frames, i.e., doubles the sleep interval, and remains in the sleep mode during the 4 frames.
Accordingly, the sleep interval increases from the initial-sleep window value to the final-sleep window value, and such an update algorithm is called the Sleep Interval Update Algorithm.
Below, a description will now be made of messages currently defined in the IEEE 802.16e communication system for supporting the sleep mode operation and the awake mode operation described above.
(1) Sleep Request (SLP-REQ) Message
The Sleep Request message is transmitted from an MSS to a BS, and is used by the MSS to request a state transition to the sleep mode. The Sleep Request message includes parameters, or information elements (IEs), required by the MSS to operate in the sleep mode. A format of the Sleep Request message is illustrated in Table 2.
TABLE 2SYNTAXSIZENOTES  SLP-REQ_MESSAGE_FORMAT( ) {MANAGEMENT MESSAGE TYPE = 458 bits    INITIAL-SLEEP WINDOW6 bits     FINAL-SLEEP WINDOW10 bits         }
The Sleep Request message is a dedicated message transmitted to the BS, according to information identified by the Basic CID of the MSS. The information elements of the Sleep Request message illustrated in Table 2 will be described below.
Management Message Type is information indicating a type of a current transmission message, and Management Message Type=45 represents the Sleep Request message.
An Initial-Sleep Window value represents a start value requested for the sleep interval (measured in frames), and a Final-Sleep Window value represents a stop value requested for the sleep interval (measured in frames). That is, as described with reference to the Sleep Interval Update Algorithm, the sleep interval can be updated within a range between the Initial-Sleep Window value and the Final-Sleep Window value.
Herein, the listening interval represents a requested listening interval (measured in frames). The listening interval can also be represented by the number of frames.
(2) Sleep Response (SLP_RSP) Message
The Sleep Response message is a message responsive to the Sleep Request message. The Sleep Response message can be used as a message indicating whether to approve a state transition request to the sleep mode from the MSS, or can be used as a message indicating an unsolicited instruction. The Sleep Response message includes information elements required by the MSS to operate in the sleep mode. A format of the Sleep Response message is illustrated in Table 3.
TABLE 3SYNTAXSIZENOTESSLP-RSP_MESSAGE_FORMAT( ) {MANAGEMENT MESSAGE TYPE = 468 bitsSLEEP-APPROVED1 bit 0: SLEEP-MODE REQUEST DENIED1: SLEEP-MODE REQUEST APPROVEDIF(SLEEP-APPROVED == 0) {After-REQ action3 bit 000: The MSS may retransmit theMOB_SLPREQ message at any time001: The MSS shall retransmit theMOB_SLPREQ message after the time duration(REQduration) given by the BS in this message010: The MSS shall not retransmit theMOB_SLP-REQ message and wait theMOB_SLP-RSP message from the BS011:111: ReservedREQ-duration4 bit Time duration for case where After-REQ-actionvalue is 001.) ELSE (START-Frame7 bitsINITIAL-SLEEP WINDOW6 bitsFINAL-SLEEP WINDOW10 bits }}
The Sleep Response message is also a dedicated message transmitted to the BS, according to information identified by the Basic CID of a MSS. The information elements of the Sleep Response message illustrated in Table 3 will be described below.
Management Message Type is information indicating a type of a current transmission message, and Management Message Type=46 represents the Sleep Response message.
A Sleep-Approved value is expressed with 1 bit. Sleep-Approved value=0 indicates that a state transition request to the sleep mode is defined (Sleep-Mode Request Denied), and Sleep-Approved value=1 indicates that a state transition request to the sleep mode is approved (Sleep-Mode Requested Approved). That is, Sleep-Approved value=0 indicates that a state transition request to the sleep mode by the MSS is denied. In this case, the denied MSS transmits a Sleep Request message to the BS according to a condition, or waits for a Sleep Response message indicating an unsolicited instruction from the BS. For Sleep-Approved value=1, the Sleep Response message includes a Start-Frame value, an Initial-Sleep Window value, and a Final-Sleep Window value. For Sleep-Approved value=0, the Sleep Response message includes a Request-Action (REQ-Action) value and a Request-Duration (REQ-Duration) value.
The Start-Frame value is a frame value until the MSS enters a first sleep interval, excluding the frame in which the Sleep Response message has been received. That is, the MSS state transitions to the sleep mode after expiration of the frames corresponding to the start frame value from the next frame of the frame where the Sleep Response message has been received.
As described above, the Initial-Sleep Window value represents a start value for the sleep interval (measured in frames), and the Final-Sleep Window value represents a stop value for the sleep interval (measured in frames). The REQ-Action value represents an action that should be taken by the MSS, a transition request to the sleep mode from which was defined.
(3) Traffic Indication (TRF_IND) Message
The traffic indication message is transmitted from a BS to a MSS during the listening interval, and indicates the presence of packet data to be transmitted from the BS to the MSS. A format of the traffic indication message is illustrated in Table 4.
TABLE 4SYNTAXSIZENOTESTRF-IND_MESSAGE_FORMAT( ) {MANAGEMENT MESSAGE TYPE = 478 bitsPOSITIVE_INDICATION_LIST( ) {TRAFFIC HASBEENADDRESSEDNUM-POSITIVE8 bitsfor (i=0; i<num-positive; i++) {CID16 bits BASIC CID OFTHE MSS } } }
The traffic indication message, unlike the Sleep Request message and Sleep Response message, is a broadcasting message that is transmitted on a broadcasting basis. In addition, the traffic indication message is a message indicating the presence of packet data to be transmitted from the BS to a particular MSS, and the MSS determines if it will state transition to the awake mode or remain in the sleep mode after decoding the broadcasted traffic indication message for the listening interval.
If the MSS state transitions to the awake mode, the MSS detects frame synchronization. If an expected frame sequence number is not detected, the MSS can request retransmission of lost packet data in the awake mode. However, if the MSS fails to receive the traffic indication message for the listening interval or a positive indication is not included in the traffic indication message even though the traffic indication message is received, the MSS returns to the sleep mode.
A description will now be made of information elements of the traffic indication message illustrated in Table 4.
Management Message Type is information indicating a type of a current transmission message, and Management Message Type=47 represents the traffic indication message. Positive_Indication_List includes Num-Positive indicating the number of positive subscribers and CIDs of the positive subscribers. That is, the Positive_Indication_List represents the number of MSSs to which packet data is to be transmitted, and CIDs thereof.
Transitioning
FIG. 3 is a signaling diagram illustrating a process of state transitioning to an awake mode by a MSS under the control of a BS in the IEEE 802.16e communication system. Referring to FIG. 3, an MSS 300 arrives at a listening interval at Step 311. If there is traffic, or packet data, to be transmitted to the MSS 300, a BS 350 buffers the packet data, and transmits a traffic indication message to the MSS 300 at Step 313.
Here, the traffic indication message includes the information elements described in connection with Table 4. The MSS 300 receiving the traffic indication message from the BS 350 determines if there is the positive indication in the traffic indication message. If there is the positive indication, the MSS 300 reads a Basic CID included in the traffic indication message and determines if its own Basic CID is included in the traffic indication message. If it is determined that its own Basic CID is included in the traffic indication message, the MSS 300 state transitions from the current mode, i.e., the sleep mode, to the awake mode at Step 315.
FIG. 4 is a signaling diagram illustrating a process of state transitioning to a sleep mode and maintaining the sleep mode by an MSS under the control of a BS in the IEEE 802.16e communication system. In FIG. 4, the MSS receives the traffic indication message for the listening interval, and then returns to the sleep mode according to a condition. In this case, if there is downlink traffic to be transmitted to several MSSs in the sleep mode state, the BS buffers the traffic for the MSSs, and includes Basic CIDs designating the corresponding MSSs in a periodically transmitted BS traffic indication message, before transmission on a broadcasting basis when the MSSs arrive at the listening interval.
Referring to FIG. 4, if a MSS 400 awaken in a listening interval 411 at Step 411, and receives a traffic indication message from a BS 450 at Step 413, the MSS 400 determines if its own Basic CID is included in the received traffic indication message. Here, because the MSS 400 fails to detect its own Basic CID from the BS traffic indication message, the MSS 400 continuously determines for the listening interval if its own Basic CID is included in received BS traffic indication messages 415 and 417. The MSS 400 continuously repeats the above process for the listening interval. If the MSS 400 stays in the Negative Basic CID state until the listening interval expires at Step 419, the MSS 400 returns to the sleep mode at Step 421.
As described above, the MSS 400 maintains the sleep mode for a doubled sleep interval, and then repeats the above process when it arrives again at the listening interval. However, if the MSS 400 detects a Positive Basic CID, the MSS 400 state transitions to the awake mode as described in connection with FIG. 3.
FIG. 5 is a diagram illustrating an operation of updating a sleep interval in a sleep mode by an MSS under the control of a BS in the IEEE 802.16e communication system. In FIG. 5, an MSS 570 receives a traffic indication messages transmitted by a BS 501 on a broadcasting basis for listening intervals 543, 547, and 551, and when Negative Basic CIDs 519, 529, and 539 are included in the received traffic indication messages, the MSS 570 doubles the sleep intervals 541, 545, and 549, and then returns to the sleep mode. If the MSS 570 detects a Positive Basic CID for the listening intervals 543, 547, and 551, the MSS 570 state transitions to the awake mode as described in conjunction with FIG. 3.
A format of the traffic indication message transmitted by the BS on a broadcasting basis to enable the MSS to state transition to the awake mode for the listening interval is illustrated in FIG. 6.
FIG. 6 is a diagram illustrating a format of a traffic indication message transmitted from a BS to a MSS in the IEEE 802.16e communication system. Referring to FIG. 6, a traffic indication message 600 includes MAC frame header parts 611 and 613 indicating that a corresponding transmission message is a traffic indication message, and traffic indication index parts 615, 617, and 619 indicating the contents of an actual traffic indication message.
The MAC frame header parts 611 and 613 include a Management Message Type field 611 indicating a type of the transmission message and a Num-of-Positive field 613 indicating a length of a traffic indication message. Herein, because the message is a traffic indication message, a value of 47 is stored in the Management Message Type field 611.
To enable three MSSs to simultaneously state transition to the awake mode through the traffic indication message 600, it is necessary to make CIDs for the three MSSs with traffic indication indexes. Therefore, in order to instruct the three MSSs to make a state transition to the awake mode, a value of 3 is stored in the Num-of-Positive field 613 and Basic CIDs for the three MSSs are included in the next fields before being transmitted. For example, in order to instruct first to third MSSs (MSS#1, MSS#2, and MSS#3) 621, 623, and 625 to state transition to the awake mode, Basic CIDs 615, 617, and 619 for the MSSs should be stored. Because the Basic CID includes 16 bits, or 2 bytes, a 6-byte data field is needed to instruct three MSSs to state transition to the awake mode.
As described above, the traffic indication message 600 is a broadcasting message, and all MSSs in their listening interval among the MSSs belonging to a particular BS receive the traffic indication message 600. The MSSs determine if their own Basic CIDs are included in the traffic indication message 600, to thereby determine whether they will maintain the sleep mode or make a state transition to the awake mode.
Above, a description has been made of the sleep mode operations proposed in the current IEEE 802.16e communication system. Next, a description will be made of problems of the sleep mode operations.
(1) In the IEEE 802.16e communication system, if there is traffic to be transmitted to MSSs in the sleep mode, the BS includes 16-bit Basic CIDs designating the corresponding MSSs in the traffic indication message as described above. However, a range of Basic CIDs designating MSSs in one BS occupies a very small part of CID#1 to CID#m among a total of 65536 CIDs. Therefore, 16-bit CIDs necessary for identifying MSSs include unnecessary most significant bits (MSBs).
As the number of MSSs that can be managed by the BS increases, the number of Basic CIDs that can be included in the traffic indication message in the above-described method also increases according thereto. For example, if the number of MSSs that can be managed by one BS is 30, only 5 bits are needed in indicating all of the MSSs. However, the conventional IEEE 802.16e communication system uses 16-bit CIDs as usual. For this, the traffic indication message needs a Basic CID group of a maximum of 60 bytes (30×2 bytes), or 480 bits.
In addition, the IEEE 802.16e communication system needs a specific bandwidth in order to transmit a traffic indication message to the MSSs, and as the number of MSSs that can be managed by one BS increases, the maximum size of the traffic indication message also increases according thereto, causing an increase in the bandwidth in use. Therefore, in order to minimize an influence on the bandwidth for transmitting data traffic, Basic CIDs for enabling the MSSs in the sleep mode to make a state transition to the awake mode are separately transmitted with several traffic indication messages. As a result, the listening interval for which the MSS receives the traffic indication message is also increased, causing unnecessary power consumption.
(2) In the IEEE 802.16e communication system, a MSS in the sleep mode awakes for the listening interval and repeats a process of waiting for a traffic indication message transmitted by the BS and determining if there is a Basic CID indicating the MSS in the traffic indication message. That is, if the MSS fails to receive a traffic indication message for the listening interval or there is no Basic CID in the traffic indication message even though the traffic indication message is received, the MSS continues to perform the above process. Therefore, the BS is not required to compel even the MSS remaining in the listening interval to make a state transition to the awake mode based on service scheduling for which load balancing on all MSSs is taken into consideration. However, an MSS, which is not informed about the situation, waits for a traffic indication message, continuously and unnecessarily wasting its power until expiration of the listening interval. Accordingly, there is a demand for various algorithms for directing the MSS to return to the sleep mode before expiration of the listening interval, thereby minimizing power consumption.